Regional anesthesia

Test dose

Another factor to ensure patient safety is the use of test doses. A test dose is performed through the use of a marker medication that will cause easily observed changes in a patient’s vital signs when injected intravascularly. This can be an important tool, as the lack of aspiration of blood is not a reliable indicator that the needle is extravascular. The typical marker medication is epinephrine (0.5 mcg/kg, max 15 mcg), though isoproterenol has been used as well. Defined parameters for a positive test dose while under general anesthesia with sevoflurane have been published. These parameters include an increase in T-wave amplitude of 25% or more, increased blood pressure of 15 mm Hg or more, and an increase in heart rate of 10 bpm or more ( ; ; ; ). Changes in the T-wave amplitude in lead II of the ECG have been shown to be as effective as an increase in heart rate ( ). Anticholinergics (atropine or glycopyrrolate) increase the sensitivity of the test dose when it is performed in an anesthetized patient. Others have shown that the change in T-wave amplitude in lead II of the ECG was more reliable than the increase in heart rate ( ; ). These hemodynamic variables may not be as reliable when a patient is anesthetized using a total intravenous anesthetic. During these anesthetics, Polaner and colleagues found that blood pressure was the most consistent finding, as evidenced by an increase of at least 30% within 2 minutes. Heart rate changes were less reliable, and T-wave changes were not found to be a reliable indicator ( ).

Treatment

The American Society of Regional Anesthesia and Pain Medicine (ASRA; https://www.asra.com ) recommends that each institution that performs regional anesthesia have 20% lipid emulsion immediately available and the means for its rapid delivery. The ASRA has published a step-by-step process for treating LAST ( Fig. 24.1 ).

It is important to avoid exacerbating disturbances such as hypoxia, hypoventilation, and tissue acidosis. Acidosis causes a decreased affinity of lipid rescue solution for local anesthetics, though the capacity remains unchanged ( ). Propofol should not be used, as the lipid content is too low to be of use and administration could potentiate hypotension. Benzodiazepines can be administered for seizure prevention. It is also best to avoid vasopressin, calcium channel blockers, beta blockers, and other local anesthetics. During resuscitation, the dose of epinephrine should be reduced to 1 mcg/kg instead of 10 mcg/kg. In rats, 10 mcg/kg of epinephrine decreased the chances of successful resuscitation from bupivacaine toxicity with lipid emulsion as evidenced by increased lactate, worsened acidosis, and poor recovery at 15 minutes ( ). Because LAST can recur, Marwick and colleagues recommended monitoring the patient for at least 12 hours ( ). Risks associated with administering lipid emulsion ( ) include infection, thrombophlebitis, altered inflammatory response, allergic reactions, embolization of emulsified fat particles, warfarin resistance, interference with ECMO circuits, increased intracerebral pressure after traumatic brain injury, weakness, altered mental status, and seizures.

Though many potential risks are associated with lipid rescue, nonetheless, lipid emulsion is still the single best treatment for systemic local anesthetic toxicity.

Bupivacaine

Bupivacaine is a racemic mixture of equimolar amounts of the enantiomers R(+) bupivacaine and S(−) bupivacaine. Pharmacokinetic studies of bupivacaine injected in the caudal space or epidural space (as noted in Table 24.3 ) have demonstrated differences between infants and children ( ; ; ). Infants have a greater volume of distribution (3.9 vs. 2.7 L/kg), an increased elimination half-life (7.7 vs. 4.6 hours), and decreased clearance (7.1 vs. 10.0 mL/kg per minute) compared with older children.

These early cases of toxicity prompted an editorial by Berde, who addressed the false generalization that children were more resistant to local anesthetic toxicity than adults ( ). Berde recommended that the maximum allowable dosages of local anesthetics should not be exceeded (see Table 24.1 ), that infusion rates should be reduced in children with risk factors for seizures, and that the maximum allowable dosages should be reduced by at least 30% for infants younger than 6 months of age ( ).

Ropivacaine

Ropivacaine has become a commonly used local anesthetic in pediatric patients, with onset times similar to bupivacaine and durations of actions that are similar or perhaps slightly longer than bupivacaine ( ; ; ; ). Ropivacaine has a lower risk for CNS and cardiac toxicity than bupivacaine. In a pharmacokinetic study of ropivacaine in children ages 1 to 8 who were administered 1 mL/kg of 0.2% ropivacaine for caudal block, the free plasma concentrations were well below toxic levels ( ). The pharmacokinetics are noted in Table 24.3 . When infants younger than 3 months of age were compared with infants 3 months to 1 year of age, the maximum free concentration of ropivacaine was significantly higher in the younger age group (99 mcg/L versus 38 mcg/L). The total and free plasma concentrations in all of the children younger than 1 year of age were within the range of concentrations previously reported for adults and older children ( ; ; ).

Epidural ropivacaine in children 1 to 9 years of age was studied to determine pharmacokinetics and safety of 24- to 72-hour infusions ( ). After a 2-mg/kg bolus, an infusion of 2 mg/mL at 0.4 mg/kg per hour was delivered and resulted in plasma concentrations that remained stable throughout the infusion and were well below toxic levels.

Levobupivacaine

Levobupivacaine is the S(−) enantiomer of bupivacaine and is less toxic to the CNS or heart than the racemic bupivacaine ( ; ; ). In pediatric studies, levobupivacaine resulted in wide ranges of plasma concentrations. In children younger than 2 years of age, Chalkiadis and colleagues found that the time to peak plasma concentration ranged between 5 and 60 minutes and was later in children younger than 3 months of age ( ).

Epinephrine

Epinephrine is a commonly used adjunct and the only adjunct added by drug manufacturers to their marketed local anesthetic preparations. Typically, it has been used in a concentration of 5 mcg/mL (1:200,000), with the intent of identifying inadvertent intravascular injection (test dose) and/or prolonging local anesthetic action. The test dose of epinephrine is typically limited to 0.5 mcg/kg (0.1 mL/kg of a 1:200,000 solution) to a maximum dose of 15 mcg.

A positive test dose after intravenous injection of 0.5 to 1 mcg/kg of epinephrine is defined as an increase in heart rate of 10 to 20 bpm, a 25% change in T-wave amplitude, ST segment changes on ECG, or a 10% increase in systolic blood pressure. Polaner and colleagues suggested that blood pressure changes are the most sensitive finding in patients undergoing total intravenous anesthesia ( ). During caudal administration, epinephrine does prolong the action of short-acting local anesthetics but has less effect on long-acting local anesthetics such as bupivacaine or ropivacaine. Epinephrine has been demonstrated to decrease C _max without affecting time to peak concentration ( ). Its effects are most profoundly seen in neonates and infants and only to a limited degree in older children ( ). In 2012 a case series describing devastating neurologic complications after epidural analgesia in four children raised concern regarding the use of epinephrine in the epidural space ( ). Editorial recommendations included limiting the dose of epinephrine within the epidural space to a test dose of 0.5 mcg/kg in 0.1 mL/kg ( ).

Opioids

Opioids have been widely used as adjuncts to neuraxial blocks for many years and are still commonly used today. Although their synergistic effect when combined with local anesthetics has been repeatedly demonstrated, the optimal choice of opioid still gives rise to both debate and confusion. There is a widespread misconception that neuraxial opioids produce analgesia exclusively by a selective spinal mechanism. This is simply not true; in fact, uptake into the systemic circulation with subsequent redistribution to brainstem opioid receptors is commonly the mechanism by which analgesia is achieved. The reason for this is the relatively long distances neuraxial opioids must diffuse to access opioid receptors in the spinal cord dorsal horn. In the process, they can partition into tissues other than the spinal cord or be cleared into plasma, both of which decrease the amount of drug available to bind to spinal opioid receptors ( ).

Spinal bioavailability of epidurally administered opioids is dependent on the complex interaction of a number of factors, including redistribution from the epidural space to surrounding tissues, meningeal permeability, dural blood flow, and CSF spread ( ). Consequently, when administered by epidural infusion, the spinal bioavailability of hydrophilic opioids (e.g., morphine, diamorphine, hydromorphone) is superior to that of hydrophobic opioids (e.g., alfentanil, fentanyl, sufentanil). Fentanyl may be the most common lipid-soluble opioid used in the epidural space. Epidural fentanyl infusion produces analgesia by plasma uptake and redistribution to brain/peripheral opioid receptors, whereas an epidural fentanyl bolus produces analgesia by a selective spinal mechanism ( ). These findings are likely explained by the fact that a fentanyl bolus results in a much larger amount of fentanyl in the epidural space than occurs at any single time point during a fentanyl infusion. Thus even though only a small fraction of the administered fentanyl is able to reach spinal cord opioid receptors, in the case of the fentanyl bolus, this small fraction is sufficient to produce a brief, spinally mediated analgesic effect ( ).

Alpha ₂ agonists

The analgesic effects of epidural clonidine were first demonstrated in 1984 ( ). Since then, numerous studies have demonstrated the efficacy of clonidine in humans. Epidural doses of 1 to 2 mcg/kg are effective in prolonging the action of local anesthetics and enhancing their effects ( ). Independent of dose, clonidine extends the duration of single-shot caudal analgesia by about 4 hours ( ). Higher doses are associated with increased risk for sedation, bradycardia, and hypotension. Based on three reports where a 100-fold overdose of caudal clonidine was administered to children (14 months to 5 years of age), the margin of safety for clonidine does seem to be wide. Somnolence was observed in all three patients, without cardiorespiratory consequences ( ). However, in some case reports of neonates and infants younger than 3 months of age, clonidine is not recommended because of possible increased risk for respiratory depression or apnea. The immature respiratory system in preterm infants may put them at higher risk for clonidine-induced respiratory depression ( ).

The analgesia derived from epidural infusion of dilute local anesthetics is improved by the addition of clonidine (0.1 mcg/kg/hr). Additionally, the use of clonidine rather than opioids for this purpose eliminates the risk for opioid-related side effects such as pruritus and nausea ( ).

Clonidine as an analgesic adjuvant to peripheral nerve blocks is controversial. As α ₂ -adrenergic receptors are not found in peripheral nerves, the mechanism of action for clonidine in this setting is limited to local vasoconstriction or via an alternate, non-α-adrenoreceptor effect altogether ( ). More specifically, clonidine, and the more superselective α ₂ blocker dexmedetomidine, prolongs the duration of analgesia by blocking the so-called hyperpolarization-activated cation current ( ). This results in prolonged hyperpolarization of the nerve and an analgesic action. This may also produce a selective sensory effect, as this action appears to be more pronounced in C fibers (pain) than in A-alpha fibers (motor).

Early studies suggested that lidocaine onset times were reduced, and analgesia was prolonged. However, subsequent studies have been less convincing. A systematic review of 27 double-blind randomized controlled trials in adults found mixed results. Clonidine appeared to be more effective when added to local anesthetics for upper-limb blocks than in lower extremities ( ). Pediatric studies are limited, but a retrospective pediatric audit found that the sensory block of dilute (≤0.2%) bupivacaine or ropivacaine was extended by a few hours and that the incidence of motor block was increased compared with local anesthetic alone ( ).

Clonidine may be viewed as the most useful adjunct for regional anesthesia in children ( ). A single dose of 1 mcg/kg has been recommended for caudal, spinal, or perineural administration, whereas Lönnqvist recommends an infusion rate of 0.1 mcg/kg/hr for epidural or perineural administration ( ). Unintentional intraneural injection of local anesthetic during the performance of peripheral nerve blocks still occurs despite the use of ultrasound guidance. Although this rarely results in permanent sequelae, significant nerve injury may occur. In this setting, the adjunct use of clonidine may be advantageous, as animal studies have shown a protective effect when administered at the site of injury ( ).

Dexmedetomidine is a highly selective α ₂ agonist that has demonstrated promising results when used in the neuraxis or for peripheral nerve blocks. Furthermore, evidence exists that dexmedetomidine has antiapoptotic properties and might even have a future role as a neuroprotective agent ( ). Data in children is limited at this time, but a meta-analysis of six pediatric studies (328 patients) demonstrated a significantly longer duration of caudal analgesia (time to first analgesic requirements) in patients receiving dexmedetomidine (1 to 2 mcg/kg) with local anesthetic (bupivacaine or ropivacaine) compared with local anesthetic alone. Side effects in the two groups were comparable without significant differences in hemodynamic changes or emergence time. The authors concluded that dexmedetomidine as an additive to local anesthetic provides a significantly longer postoperative analgesia with comparable adverse effects and hemodynamic changes compared with local anesthetics alone, but data on the effects of different concentrations of dexmedetomidine is currently insufficient. In caudal blocks, 1 mcg/kg of dexmedetomidine had similar effects as 2 mcg/kg ( ).

No data are currently available for dexmedetomidine in the context of peripheral nerve blocks in children, but adult volunteer data show that adjunct administration of dexmedetomidine prolongs the duration of ulnar nerve block by 60% ( ).

Neostigmine

Adjunctive neostigmine (1 to 4 mcg/kg) has been shown to prolong analgesia for 9 to 10 hours after single-shot caudal block. This action is presumably due to a muscarinic effect at the level of the spinal cord. However, caudal neostigmine is associated with a 30% incidence of postoperative nausea and vomiting ( ).

Ketamine

Ketamine is a noncompetitive NMDA and mild mu-receptor agonist that has proved effective as an adjunct for caudal analgesia in doses similar to those used for intravenous pain relief (0.25 to 1 mg/kg). The most valid argument for a spinally mediated effect is that the duration of postoperative analgesia achieved with preservative-free S-ketamine is substantially longer than the effect of a similar intramuscular dose ( ). Higher doses than those mentioned earlier result in unwanted side effects (sedation, hallucinations, nystagmus, nausea, and vomiting) without additional analgesic benefit. Age-dependent neurotoxic effects of intrathecal ketamine have been demonstrated in animal studies ( ). Although the clinical relevance of these findings remains undetermined, the result has been that ketamine is no longer recommended as a local anesthetic adjunct in a number of countries.

Steroids

Dexamethasone has attracted interest as a local anesthetic adjunct for regional anesthesia. Its activity is thought to be mediated by attenuating the release of inflammatory mediators, reducing ectopic neuronal discharge, and inhibiting potassium channel–mediated discharge of nociceptive C fibers ( ). Early results seem promising, especially when mixed with other adjuncts ( ). A meta-analysis of nine trials (801 adults) demonstrated that dexamethasone prolonged analgesic duration for long-acting local anesthetics from 730 to 1306 minutes and intermediate-acting local anesthetics from 168 to 343 minutes. Motor block was prolonged from 664 to 1102 minutes ( ). Notably, Desmet and colleagues demonstrated in a double-blind, randomized, placebo-controlled study of adults undergoing shoulder surgery an equivalent prolongation of perineural administered dexamethasone with systemic administration of dexamethasone ( ).

Dexamethasone is not approved for perineural administration by the United States Food and Drug Administration (FDA), Health Canada (HC), the European Union (EU), or any other regulatory body. Finally, basic cytotoxicity research shows more neuronal death with higher concentrations of dexamethasone when combined with clinically relevant concentrations of local anesthetics ( ). In view of the lack of pediatric studies coupled with the basic science studies and clinical adult studies, the adjunct use of dexamethasone in pediatric regional anesthesia should be restricted to clinical trials ( ). The effectiveness of perineural adjuncts is still in question and will need further trials to clarify their role. It is important to note that their use is off-label and not likely to be approved by the FDA for this purpose ( ).

Physics

Understanding ultrasound requires a basic understanding of the physics of sound waves. Sound is a result of energy that is created by waves of pressure transmitted through matter such as air, liquid, or tissue. An ultrasound wave is created through the mechanical deformation of specific crystals by an electric field that causes oscillations. These oscillations project a high-frequency sound, or an ultrasound wave. This phenomenon is called the piezoelectric effect. The crystals in an ultrasound probe are piezoelectric crystals. Medical ultrasound has a frequency between 2 and 13 MHz, with a speed in tissue of around 1500 m/s. The average wavelength for medical ultrasound is less than 1 mm, which makes structures smaller than this difficult or impossible to accurately define. Most structures that need to be visualized, such as nerves for a regional technique or major vasculature, are large enough to be easily defined.

To generate an image, an ultrasound wave is projected from the probe and transmitted through the desired medium. When the sound wave encounters a structure, the wave is reflected backward to the probe. The amount of the ultrasound waves, or the strength of signal, determine how bright the displayed signal is on the imaging screen. The round-trip time from projection, reflection, and reception determines the depth of the target structure. The echogenicity of an image is used to describe its brightness on the screen. If an image is the same shade as the rest of the medium, the image is said to be isoechoic. If a structure is hyperechoic, the image is brighter than the surrounding structures. Further, a hypoechoic image is darker than the surrounding medium. If there is an absence of reflected waves, the area is anechoic. Examples of each are listed in Table 24.4 . The appearance of structures of interest are listed in Table 24.5 .

Terminology

The speed at which an ultrasound beam travels through a medium is the acoustic velocity. This differs depending on the tissue and has an effect on the image that is displayed. The stiffness of a material is its resistance to compression, which affects the propagation speed. The stiffer a substance, the faster the propagation speed. The resistance to a wave transmitted through a substance is defined by its acoustic impedance.

The frequency of a probe will determine the depth of penetration and the resolution of the image. A high-frequency probe will have a higher resolution, but it will not penetrate as far as a low-frequency probe. The depth of penetration is affected by the attenuation coefficient. This is used to estimate the amount of signal loss in a certain substance as a function of ultrasound frequency.

Axial resolution determines how well structures can be distinguished in superficial and deep planes—that is, one structure above the other. Lateral resolution determines how well structures can be distinguished when they are beside each other. Temporal resolution is important in observing objects that are in motion, such as heart valves.

Structures, especially nerves, display an angular dependence for best resolution. In other words, tilting the transducer may improve or diminish the image of the structure. The best images are produced when the structures are at right angles to the beam, maximizing the amount of signal returning to the probe. This property is known as anisotropy.

Modes

As the technology of ultrasound has improved over the years, different modes have been developed and used in medicine. A-mode displays the echoes received from the tissue as a series of spikes that correspond to the depth of the reflecting target. This is the oldest mode of ultrasound and is not applicable to regional anesthesia. B-mode, the most commonly used mode, creates a two-dimensional picture. The signal received from the tissue is represented by a dot, whose brightness is determined by the amplitude of the signal. The time that it takes to send and receive a signal (the round-trip time) determines the depth of the dot on the display. The quality of the image is, at least in part, determined by the frame rate. M-mode, which is used commonly in echocardiography, is used for imaging structures that are rapidly moving, such as heart valves, with a single scan line that can be displayed in a wave-like manner. Doppler ultrasound provides a color representation of flow using the physics of the Doppler effect. Commonly, red and blue coloration provides information on the direction and velocity of flow. This mode can be used to confirm the presence of blood vessels when scanning for a target nerve and confirm the position of a catheter tip by injecting through the catheter to create a signal.

Artifacts

An ultrasound machine emits and receives signals to generate images in real time. Though it may seem to be a continuous video, what the practitioner is viewing is actually a series of images displayed in rapid succession. Artifacts are commonly seen during ultrasonography, which can alter the image displayed on the view screen. The most common artifact is the contact artifact, which is a dropout of the image due to a loss of the acoustic coupling medium between the skin and the transducer ( Fig. 24.2 ). A reverberation artifact is the repeated reflection of the image of a highly reflective surface, such as an echogenic needle ( Fig. 24.3 ). Acoustic shadowing is an area devoid of image due to a highly reflective surface, such as bone, through which the ultrasound waves cannot pass ( Fig. 24.4 ). A propagation speed artifact, also known as a bayonet artifact, occurs when the medium through which the ultrasound beam travels changes, subsequently changing its speed. This change causes a bending of the image on the ultrasound screen much like the bending of light changes the image of a straw placed in water ( Fig. 24.5 ).

Probes

There are many different probes used in medical ultrasound. Each uses piezoelectric crystals to produce an ultrasound beam. A probe alternates between generating a beam and receiving the reflected signals. There are many manufacturers providing various tools to improve ultrasound technique. In general, ultrasound probes come in various frequencies that provide a wide selection of resolutions and penetration. In general, ultrasound probes for regional anesthesia use a frequency between 3 and 15 MHz. A high-frequency probe, such as the 13 to 6 MHz, will provide excellent superficial resolution but will not penetrate very deeply. Conversely, a low-frequency probe, such as many of the curvilinear probes, will penetrate much farther at the expense of resolution. It is important to be familiar with the selection of probes available to increase the chances of a successful regional technique. For example, when imaging superficial structures like the interscalene or supraclavicular nerve, a probe with a frequency higher than 7 MHz is preferred. Probes with frequencies higher than 10 MHz will provide better resolution, but the tissue penetration may be limited to 2 to 3 centimeters. The sciatic nerve in the subgluteal region or popliteal fossa requires deeper tissue penetration, and frequencies lower than 7 MHz are required. The footprint, or length and width, of a probe varies, with the curvilinear probes generally having a larger surface area. A smaller footprint is necessary for smaller patients and areas such as the supraclavicular fossa.

Needles

There are multiple needles on the market available for regional anesthesia. Almost any needle can be used with ultrasound to place a block, though some are more echogenic than others. A common problem with needle visualization is when the needle is nearly parallel to the ultrasound beam; few reflections are sent back to the probe to produce an image. In an attempt to facilitate visualization during block placement, manufacturers have altered needles with microscopic glass beads, reflectors, and etching ( ). These alterations are meant to send more ultrasound reflections back to the probe, even at steep angles. Most needles that are marketed as hyperechoic can provide a more echogenic image ( ). Catheters and their placement are covered elsewhere in this chapter.

With an understanding of the principles of ultrasonography and their application, as well as appropriate equipment and technique, one can provide reliable and effective regional anesthesia. The clarity of the ultrasound image, needle visualization, and type of catheter affect the rate of success and associated side effects ( ; ).

Needle handling and tip localization

Two simple nomenclatures describe needle advancement in relation to the ultrasound transducer. Out-of-plane or cross-sectional technique positions the needle transverse to the observed image. Although this approach offers the shortest skin-to-nerve distance, with a potential benefit of decreased patient discomfort, the needle shaft and tip are not clearly visualized and are deduced by transducer tilting or alignment and by tissue movement. In-plane or in-line technique advocates longitudinal advancement of the needle. This allows complete shaft/tip imaging but requires training, may create reverberation artifact, and may be inappropriate for continuous catheter placement. Hydrolocation refers to injection of a small amount of liquid (0.5 mL) to find the needle tip on the ultrasound image. Needle visualization requires considerable training and is often a source of frustration for beginners. It is prudent to adjust the image by manipulating the transducer rather than by blindly advancing the needle. When describing the position of the ultrasound probe in relation to the body, there are three spatial planes to consider. These planes are described in Box 24.4 .

Traditional landmark-based technique

After induction of general anesthesia, the patient is positioned in left or right lateral decubitus position with knees and hips flexed. When performing the block in awake neonates, prone positioning may be preferred. Sterile preparation of the skin with chlorhexidine is preferred, taking care to swab in a cranial to caudal direction. Sterile technique should be maintained throughout the procedure, as with any neuraxial technique.

The posterior superior iliac spines (PSIS) and sacral hiatus are palpated ( Fig. 24.6 ). An equilateral triangle with the line between the two PSIS as its base will have the sacral hiatus at its apex. The line between the PSIS is at S1–S2, the level of the termination of the dural sac, giving the proceduralist a general idea of the limit of needle insertion before dural puncture is likely.

The sacral hiatus is located just cephalad to the gluteal cleft and the coccyx. It is formed from the nonunion of the S5 vertebral arch. The bony protuberances of the sacral cornu can be palpated on either side of the sacral hiatus. With the nondominant gloved hand, the sacral hiatus is palpated, and with the dominant gloved hand, a needle is inserted in the midline between the sacral cornu, at a 45- to 60-degree angle with the skin. As the needle passes through the sacrococcygeal ligament, a characteristic “pop” or loss of resistance to needle insertion can be appreciated. After passing through the sacrococcygeal membrane, the needle angle is typically reduced to 30 degrees with the skin, and the needle is advanced no further than 5 mm to avoid puncturing the dural sac or a blood vessel ( Fig. 24.7 ).

Commonly employed needles are 22 G B-bevel caudal needle, 20 or 22 G intravenous (IV) catheters, and 23 G butterfly needles ( ). If an IV catheter is used, it can be threaded off the needle into the caudal epidural space.

For single-shot caudal blocks, dosing with 1 mL/kg of 0.2% ropivacaine will reliably result in a block to the level of the umbilicus. If just sacral dermatomes require coverage, then a reduced dose of 0.5 mL/kg will suffice ( Fig. 24.8 ). Ropivacaine 0.2% has largely supplanted bupivacaine 0.25% as the most widely used local anesthetic and provides reliable analgesia for 4 to 8 hours when used without adjuncts. The local anesthetic should be injected incrementally, interrupted with frequent aspiration, to assess for unintentional intravascular, intraosseous, or intrathecal injection. Palpation for induration just cephalad to the point of injection should be performed during and after injection to assess for unintentional subcutaneous or subdermal injection of local anesthetic. Continuous pulse oximetry and ECG, with frequent blood pressure checks, should be performed.

Caudal block confirmatory testing

In 1992, Lewis and colleagues described a technique of injecting 2 mL of air into the caudal space of adults, resulting in a characteristic “whoosh” sound auscultated over the thoracolumbar spine using a stethoscope. They standardized the “whoosh test” against epidurogram and found no false positives. This confirmatory test for caudal injection was later applied to pediatrics. reported the case of a probable venous air embolism in a 26-month-old, 11-kg boy after “a small volume of air” was injected as part of confirmatory testing while performing a caudal block. The “swoosh test” then emerged as a modification of the original whoosh test, wherein the injection of air was avoided by performing auscultation over the lumbar spine during injection of local anesthetic into the caudal space. found the swoosh test to be as effective a clinical predictor as the whoosh test. The use of real-time ultrasound guidance for caudal block (see later) has also emerged as a confirmatory testing technique, as needle entry and local anesthetic spread within the caudal space can be visualized in real time.

Epinephrine test dose

The epinephrine test dose adds safety to the procedure in ruling out (or in) an intravascular injection. Because the majority of pediatric patients are under general anesthesia when the block is performed, detection of intrathecal injection with a test dose of lidocaine is less useful. Achieving the recommended test dose of 0.5 mcg/kg with standard epinephrine solutions available in most operating rooms can be difficult for the busy anesthesia provider and can risk drug errors and potentially be a source of contamination for sterile epidural solutions. It is preferable to use prepared epidural test dose solutions (i.e., 1.5% lidocaine with 1:200,000 epinephrine) for this purpose.

Ultrasound-guided technique

Ultrasound guidance has emerged as a technique to more accurately identify the sacral hiatus than the traditional landmark technique and can confirm local anesthetic injection into the caudal epidural space ( Fig. 24.9 A–D). One of the most common complications of the caudal block is block failure ( ), leading to inadequate analgesia. Ultrasound for the caudal block has the disadvantage of increased cost, both in terms of increased time to perform the procedure and in the cost of the ultrasound machine and sterile probe covers required. Ultrasound is most effective with children under 6 months to 1 year of age because of incomplete ossification of bony structures improving beam penetration. But the technique has also been used to increase success of caudal blocks in older children. demonstrated a 90% success rate in U.S.-guided caudal blocks in children and adolescents weighing 30 to 50 kg. The use of ultrasound guidance has increased the success rate of caudal blocks, even with experienced practitioners ( ; ).

In short axis, the sacral cornu are identified as two inverted U-shaped structures on either side of the sacral hiatus—the characteristic “frog eyes” sign (see Fig. 24.9 A). The sacrococcygeal ligament will appear between the cornu as a hyperechoic band, with a deeper hyperechoic band beneath it that has an associated bone shadow. The latter represents the dorsal surface of the sacral canal. A needle is advanced using out-of-plane technique between the sacral cornu and through the sacrococcygeal ligament. Once the needle passes through the sacrococcygeal ligament, the probe can be turned 90 degrees to visualize the needle in-plane as it is inserted into the sacral canal ( Fig. 24.10 ). As local anesthetic is injected, an expanding hypoechogenic mass will be seen within the sacral canal, with associated dural movement within the space. Visualization of the needle within the sacral canal or spread of local anesthetic within the canal confirms successful block placement.

Technique

Anesthetized children are placed in lateral position with hips and knees flexed, and the spine kyphotic ( Fig. 24.11 ). Older children and adolescents may have epidurals placed in sitting position while lightly sedated. The skin is sterilized with chlorhexidine and draped. Sterile technique must be maintained throughout the procedure. A midline approach is generally employed in prepubescent children.

A 5 cm 20 G Touhy needle with a 24 G epidural catheter is generally used for patients under 12 months of age. A 5 cm or 9 cm 18 G Touhy needle with a 20 G epidural catheter can generally be used for patients over 12 months of age. Loss of resistance (LOR) is the most commonly used technique for identifying the epidural space. Air or saline have been well described as LOR media, although saline is probably the preferred medium for most pediatric practitioners. Proponents of air argue that it is a more sensitive medium than saline, which can be beneficial in infants who have less fibrous ligamentum flava than older children and adults. If air is used, small volumes (<1 mL) are recommended to avoid air venous embolism, and the introduction of air bubbles into the epidural space. The use of air versus saline remains controversial.

In neonates and infants, intermittent advancement of the needle, alternating with syringe pressure, provides good needle control. Continuous needle advancement with steady syringe pressure has been safely employed with older children. The estimated depth formula proposed by of 1 mm/kg for thoracolumbar epidural space in children 6 months to 10 years of age remains a useful guide.

Once the epidural space is identified with LOR, it is further confirmed by ease of catheter placement into the epidural space. The catheter should thread into the space with little resistance. The epidural catheter should be advanced to a depth such that the tip is located in proximity to the spinal interspace corresponding to the dermatomal level of surgical incision, regardless of whether a single or multiorifice catheter is used.

Leakage of epidural infusate from the catheter insertion site is a common problem in small pediatric patients for a number of reasons. The skin-to-epidural space distance is relatively short, as children have less subcutaneous tissue than adults. The pediatric ligamentum flavum is a less resilient barrier than it is in adults. Finally, the catheter is always of a smaller gauge than the needle through which it was inserted, leaving space around the catheter for fluid to leak out. With leakage, less medication is delivered to the site. Leakage can also compromise dressing integrity and therefore catheter sterility in the postoperative period. This can necessitate the catheter being discontinued prematurely. Acryl skin adhesives (e.g., Dermabond, Ethicon, Inc., Sommerville, NJ) applied at the insertion site can reduce the risk of catheter leakage. Inserting the catheter at least two interspaces below the target dermatomal level and threading the catheter cephalad such that at least 3 to 4 cm of the catheter is inserted into the epidural space will further reduce the risk of leakage.

Point-of-care ultrasound can be used before the procedure to identify the patient’s midline, find the target interspace, and measure the depth from skin to epidural space. In small children ultrasound may be used to identify catheter tip position and confirm local anesthetic spread in the epidural space. Other methods, including radiographic ( ), nerve stimulation ( ), and ECG guidance ( ), have been described to confirm epidural catheter placement. These confirmatory techniques are becoming much less common with the advent of ultrasound.

Technique

After induction of anesthesia and securing the airway, the patient is placed in the prone position, with the head turned to the side. As part of positioning, it is essential to ensure the infant’s back is straight as viewed from the long and short axis. Any curvature of the spine, especially lumbar lordosis, will interfere with catheter advancement. Bony landmarks, including the sacral hiatus and the interspace of the surgical dermatomal level, are identified and marked. The patient’s back and sacrum are prepared with chlorhexidine and sterilely draped. The distance from caudal space to the desired interspace should be measured, and the catheter advanced the corresponding distance. An 18 G Touhy needle (alternately an angiocatheter) is inserted into the caudal space, preferably under ultrasound guidance. After injection of a small volume of saline, 0.5 to 1 mL, the epidural catheter is threaded through the Touhy needle until the premeasured distance is reached. Resistance to catheter advancement should be low, as in other techniques for epidural catheter placement. Resistance to advancement may indicate coiling of the catheter or misdirection. Confirmation of the catheter tip at the desired interspace can be accomplished with ultrasound visualization of the catheter itself or spread of hypoechoic fluid (saline or test dose solution) in the epidural space and displacement of the thecal sac downward in the spinal canal. The use of a stimulating catheter inserted caudally and advanced while observing segmental muscle stimulation has also been described ( ). Radiographic ( ), and even ECG confirmation ( ), have also been used. found that 3% of caudally threaded epidural catheters from the PRAN database could not be placed.

Complications

Complications during spinal anesthesia in children are uncommon. In the ADARPEF study, an intravascular injection during spinal anesthesia was the only reported complication in 506 cases ( ). Although considered to be a rare occurrence in children, the incidence of postdural puncture headache is actually between 10% and 50% in children 10 to 18 years of age ( ; ). Although the incidence of PDPH is less in children younger than 8 years old compared with adults, it appears to be more common in teenagers (12 to 18 years) than adults. In one observational study, the incidence was threefold higher in teenagers (4.9%) than adults (1.8%) ( ). The onset of symptoms from a postdural puncture headache typically occurs within 48 hours, with the hallmark symptom being a frontal or occipital headache that is postural. The cause of postdural puncture headache is most likely a persistent leakage of spinal fluid that causes a net decrease in CSF volume and intracranial pressure. The supine position helps to alleviate the symptoms by decreasing the effect of gravity on the CSF leak. The size of the dural perforation is the primary predictor of the development of postdural puncture headache. To reduce the risk for postdural puncture headache, smaller-gauge needles are used, and the needle is inserted with the bevel parallel to the dura’s longitudinal fibers ( ). Electron microscopy has revealed that the collagen fibers of the dura are arranged in several layers and that the thickness of the dura is more predictive of whether a leak will occur than the orientation of the needle’s bevel ( ). This explains the unpredictability of postdural puncture headache.

If a diagnosis of postdural puncture headache is made, simple measures such as bed rest and hydration can decrease the volume of CSF loss. In addition, analgesics to reduce the headache and intravenous caffeine may be administered. Caffeine is effective because of its ability to cause cerebrovascular vasoconstriction, resulting in decreased cerebral blood flow. In one study, when caffeine was used prophylactically in adults, visual analog pain scores and analgesic demand after postdural puncture headache were lower ( ). The oral dosage for adult patients is 300 to 500 mg once or twice a day ( ). Pediatric patients can be administered 10 mg/kg oral caffeine divided twice daily ( ). The intravenous formulation of caffeine is administered with sodium benzoate. The pediatric dose is 10 mg/kg, with a max dose equal to the adult intravenous dose of 500 mg. This may be repeated within 2 to 4 hours if the headache is unchanged after the first dose ( ).

If conservative measures to treat postdural puncture headache are ineffective after 24 to 48 hours, an epidural blood patch should be considered. This requires that blood be drawn sterilely from a peripheral vein. The blood is then injected into the epidural space under aseptic technique. An epidural blood patch is most effective 48 to 72 hours after the dural puncture, and it may be ineffective if performed immediately after dural tap because high leakage of CSF may interfere with blood clotting ( ). If a child is awake during the placement of an epidural blood patch, the clinician should stop the injection once the child feels either discomfort or pressure in the back. If a child is anesthetized during the performance of an epidural blood patch, no more than 0.3 mL/kg of blood should be injected into the epidural space ( ).

Total spinal block with respiratory arrest and bradycardia is another complication of spinal anesthesia ( ). The preganglionic sympathetic blockade that is commonly seen in adults secondary to a high spinal is not typically seen in children, particularly in infants ( ; ; ). Dohi and colleagues were the first to describe the lack of hemodynamic changes after spinal block–induced sympathetic blockade in children. They found that children younger than 5 years of age had little or no hemodynamic response to a T3-level tetracaine spinal anesthetic, whereas children older than 8 years of age had cardiovascular responses that were more similar to those of adults. The mechanism for this lack of hemodynamic sympathectomy was postulated to be the immaturity of the sympathetic nervous system and differences in CSF volume and spinal cord surface area. In addition, it is also possible that the smaller blood volume that is present in the lower extremities of a young child compared with that of an adolescent or adult may account for less venous pooling and therefore less hemodynamic change ( ; ). Despite the typical lack of cardiovascular compromise, neonates occasionally require ventilatory support or pharmacologic intervention because of a high spinal anesthesia with a resulting blockade of the cardiac accelerator fibers or a decrease in stimulation of the right atrial stretch receptors ( ). Investigation has shown that even former premature infants in the absence of fluid loading tolerate high spinal anesthesia with minimal autonomic changes ( ).

Anatomy

Innervating the scalp, forehead, and upper eyelid, these two nerves are terminal branches of the ophthalmic division of the trigeminal nerve, which exits the cranium through the superior orbital fissure and then divides into three branches: lacrimal, frontal, and nasociliary. The frontal branch further divides to form the supraorbital and supratrochlear nerves, which emerge through their respective notches on the orbital roof.

Dermatome distribution

These nerves innervate the frontal scalp, forehead, median portion of the upper eyelid, and the bridge of the nose.

Affected muscle group

These are purely sensory nerves with no muscle innervation.

Landmark technique

The child lies supine with his or her head in a neutral position. The ipsilateral eye should be covered with an impermeable dressing to prevent chemical injury during skin preparation. In the midline, identify, by palpation, the supraorbital foramen in the roof of the orbital rim. After chlorhexidine preparation, advance a 27- to 30-gauge needle until bony contact is made, and then withdraw 1 mm. After negative aspiration, inject local anesthetic, then massage gently and apply firm pressure to prevent hematoma formation ( Fig. 24.13 ).

Ultrasound technique

Ultrasound technique has not been described.

Dosing

Administer 1 to 2 mL 0.2% to 0.5% ropivacaine (per side).

Complications

Intravascular injection and hematoma formation are possible. Orbital injury would be a rare and entirely avoidable complication arising from lack of attention to careful technique.

Anatomy

The maxillary branch of the trigeminal nerve becomes the infraorbital nerve once it exits the skull via the infraorbital fossa just inferior to the infraorbital rim and deep to the levator labii superioris. It then divides into terminal sensory branches.

Dermatome distribution

This nerve innervates the lower eyelid and upper lip, teeth and gums, nasal mucosa, and the palate and roof of the mouth.

Affected muscle group

This is a purely sensory nerve with no muscle innervation.

Landmark technique

There are two approaches described: an extraoral and an intraoral approach. For both techniques, the child lies supine with the head in a neutral position.

Intraoral approach.

This is the preferred technique, as the risk for visible skin bruising is reduced. In the midline, palpate the infraorbital foramen in floor of orbital rim. Place a finger over the foramen to prevent needle injury to the orbit. Evert the upper lip, then insert a 27- to 30-gauge needle at the level of first premolar and advance toward the foramen. After negative aspiration, inject local anesthetic, then massage gently and apply firm pressure to prevent hematoma formation ( Fig. 24.14 ).